1. Field of the Invention
The invention relates to a digital-to-analog converter for bipolar signals, the value of the most significant bit changing when the signal passes through the zero level, which converter comprises n inputs for n bits to be converted, at least n switches, which are connected to the n inputs and which switch analog reference quantities, which together determine the analog value, which is available on an output of the converter, and means for eliminating the influence of switching transients on the analog value when the bits change in value.
2. Description of the Prior Art
Such a digital to-analog converter is known from the article by F. Maddens and M. Quintin, entitled: "Digital to analog converter for low level PCM signals" in IBM Technical Disclosure Bulletin, Vol. 17, No. 5, October 1974, pages 1383 and 1384. The article gives a solution to the problem of the poor signal-to-noise ratio at small signal values during passage of the zero level of the signal. All bits are then switched and change in digital value. If, as is customary, a switch which switches an analog quantity, is connected to each bit cell, an equal number of analog quantities can be switched on and off. Different switching times and delays give rise to switching transients on the analog output signal. The sum of said transients represents an additional signal, which is added to each changing input signal which passes through zero. For example, in the case of sine wave signals, such as audio signals, this contribution results in increased distortion and a reduction of the signal-to-noise ratio. In the known digital-to-analog converter this problem is solved by means of logic circuits arranged between the bit inputs, so that for the n bit inputs n+m switching lines are obtained which each actuate a switching transistor for switching on and off a current, which via a resistor network produces an analog voltage, which no longer has the stepped binary waveform. Thus, it is avoided that switching n bits also results in n analog quantities being switched. The digital-to-analog converter which is described is a 3-bit converter. For more bits the number of digital circuits becomes uneconomically large, while moreover a special resisitor network must be used.